The present invention relates to a packaged, solid-state surge protector and to its use in the protection of telecommunication and other electronic systems that are subject to transient current surges of up to 10.sup.4 amps or more.
Electronic systems often must be protected against transient current surges which might otherwise damage the system. Thus, power-transmission systems and telecommunications systems typically incorporate surge protectors, for example, where conductors enter a central office or a building, whether industrial, commercial or residential, and at various other positions in the system.
Conventional protectors often include a gap across which a surge current shorts to ground, i.e., the protector provides a connection to ground in parallel with the conductor it is protecting, with the gap normally providing isolation. The form of the protector varies. The gap may thus be between two carbon blocks or electrodes, and open to the atmosphere. Alternatively, in so-called "gas tube" protectors the gap may be between two electrodes sealed within a housing, the gap being at a sub-atmospheric pressure.
Surge protectors that incorporate a gap have several drawbacks, including the inherently statistical nature of current conduction across the gap, which are reflected in an undesirable instability. An open-air gap, for example, is subject to contamination which affects conduction across the gap in an unpredictable manner. A gas tube protector can leak, resulting in an increase in internal pressure and breakdown voltage.
Accordingly, it has been proposed to protect electronic systems against surges by means of a unidirectional or bidirectional semiconductor (solid-state) device wherein an avalanche-breakdown phenomenon is exploited to effect a clamp on voltage, as in a zener diode, or a regenerative feedback action, as in a thyristor. A semiconductor device of this sort is typically an overvoltage protector, and is structurally similar to standard, medium-power semiconductor devices like the thyristor.
Unlike standard semiconductor devices, however, a semiconductor overvoltage protection device must fulfill a specific role, both before and after failure, in the circuit where it is a component. In particular, after the device fails (due to a current surge) it must provide a sustained short between the conductors in the circuit of which the device is a part. Also in contrast to standard semiconductor power devices, semiconductor overvoltage protectors normally do not operate in a rapid-repetitive mode, but rather are subjected to transient surges (or to trains of transient pulses which have the effect of a single surge) which vary in duration and amplitude. By virtue of practical limits on heat transfer through a protector device, heat generated by such a transient cannot be removed from the device in a period comparable to the duration of the transient.
Thus, while the average power normally dissipated by a semiconductor protection device is very low, the instantaneous power (and the heat generated) during a transient surge can reach levels sufficient to destroy totally the active (semiconductor) region of the device, which in turn can lead to loss of contact between the conductors and the elimination of the short. Because they are prone to destruction in this fashion at a relatively low current surges, medium-power semiconductor protection devices have heretofore been limited in their use to circuits where maximum surges were well below 10.sup.4 amps.